The recombinant cell and the flooded cell are two different types of commercially available lead-acid battery designs. Both types include adjacent positive and negative electrodes that are separated from each other by a porous battery separator. The porous separator prevents the adjacent electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow between adjacent positive and negative plates.
The first type of lead-acid battery, a recombinant battery, specifically a value-regulated lead-acid (VRLA) battery, typically includes an absorptive glass mat (AGM) separator composed of microglass fibers. While AGM separators provide high porosity (>90%), low electrical resistance, and uniform electrolyte distribution, they are relatively expensive and still do not offer precise control over oxygen transport rate or the recombination process. Furthermore, AGM separators exhibit low puncture resistance that is problematic for two reasons: (1) the incidence of short circuits increases and (2) manufacturing costs are increased because of the fragility of the AGM sheets. In some cases, battery manufacturers select thicker, more expensive, separators to improve the puncture resistance, while recognizing that the electrical resistance increases with thickness.
The second type of lead-acid battery, the flooded cell battery, is characterized by absorption of only a small portion of the electrolyte into the separator. Flooded cell battery separators typically include porous derivatives of cellulose, polyvinyl chloride, organic rubber, and polyolefins. More specifically, microporous polyethylene separators are commonly used because of their ultrafine pore size, which inhibits dendritic growth while providing low electrical resistance, high puncture strength, good oxidation resistance, and excellent flexibility. These properties facilitate sealing of the battery separator into a pocket or envelope configuration in which a positive or negative electrode can be inserted.
More recently, enhanced flooded batteries (EFB) have been developed to meet the high cycling requirements in “start-stop” or “micro-hybrid” vehicle applications. In such applications, the engine is shut off while the car is stopped (e.g., at a traffic light) and then re-started afterwards. The advantage of a “start-stop” vehicle design is that it results in reduced CO2 emissions and better overall fuel efficiency. A major challenge in “start-stop” vehicles is that the battery must continue to supply all electrical functions during the stopped phase while being able to supply sufficient electric current to re-start the engine at the required moment. In such cases, the battery must exhibit higher performance with respect to cycling and recharge capability, as compared to that of a traditional flooded lead-acid battery design.
Most flooded lead-acid batteries include polyethylene separators. The term “polyethylene separator” is something of a misnomer because these microporous separators require large amounts of precipitated silica to be sufficiently acid wettable. The volume fraction of precipitated silica and its distribution in the separator generally control its electrical properties, while the volume fraction and orientation of polyethylene in the separator generally control its mechanical properties. The porosity range for commercial polyethylene separators is generally 50%-65%.
During the manufacture of polyethylene separators, precipitated silica is typically combined with a polyolefin, a process oil, and various minor ingredients to form a separator mixture that is extruded at elevated temperature through a sheet die to form an oil-filled sheet. The oil-filled sheet is calendered to its desired thickness and profile, and the majority of the process oil is extracted. The sheet is dried to form a microporous polyolefin separator and is slit into an appropriate width for a specific battery design.
The polyethylene separator is delivered in roll form to lead-acid battery manufacturers, where the separator is fed to a machine that forms “envelopes” by cutting the separator material and sealing its edges such that an electrode can be inserted to form an electrode package. The electrode packages are stacked such that the separator acts as a physical spacer and an electronic insulator between positive and negative electrodes. An electrolyte is then introduced into the assembled battery to facilitate ionic conduction within the battery.
The primary purposes of the polyolefin contained in the separator are to (1) provide mechanical integrity to the polymer matrix so that the separator can be enveloped at high speeds and (2) to prevent grid wire puncture during battery assembly or operation. Thus, the hydrophobic polyolefin preferably has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high puncture resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator web, thereby lowering the electrical resistivity of the separator. In the absence of silica, the sulfuric acid would not wet the hydrophobic web and ion transport would not occur, resulting in an inoperative battery. Consequently, the silica component of the separator typically accounts for between about 55% and about 80% by weight of the separator, i.e., the separator has a silica-to-polyethylene weight ratio of between about 2.0:1 and about 3.5:1.
In response to the increased price of lead, battery manufacturers desire a separator with even lower electrical resistance than is currently available so that they might be able to achieve the same discharge rate performance with less active material (i.e., lead and its oxides) in the electrodes. Furthermore, low electrical resistance and high porosity are beneficial to lead-acid batteries used in start-stop applications, in which acid stratification must be minimized to achieve enhanced cycle performance. As such, there is a need for a separator that provides high porosity, minimizes acid stratification, and maintains good mechanical properties in both a “wet” state (i.e., in the presence of electrolyte) and a “dry” state (i.e., in the absence of electrolyte).